Such methods and plants are used for instance when smelting ores, for example in the production of iron from iron ores, ferronickel alloys from iron-nickel ores or the like. Before heat-treated in this way the ores are reduced in a succeeding process stage. While this preheating of iron oxide containing ores previously was chiefly carried out in rotary kilns, fluidized-bed reactors have also been used for this purpose for some years.
From EP 0 222 452 B1 there is known a method for reducing metal oxides to obtain lower metal oxides by means of carbonaceous reducing agents, in which initially solids containing higher metal oxides are calcined with hot gases at 800 to 1100° C. in a first reactor in which the solids are suspended by the hot gases. The solids calcined in this way are subsequently reduced to form lower metal oxides in a second reactor with a stationary fluidized bed by adding carbonaceous reducing agents and oxygen-containing gases at a temperature of 800 to 1100° C. Calcining can be carried out in a fluidized bed which is either formed stationary or preferably circulating. However, the energy utilization of the calcining step, which is achieved by using a stationary fluidized bed, needs improvement. This is due to the fact that the mass and heat transfer is rather moderate due to the comparatively low degree of fluidization, and therefore an internal combustion is difficult to control. In addition, a preheating of solids can hardly be integrated in a suspension heat exchanger, because dust-laden gases are rather not admitted to the fluidizing nozzles of the stationary fluidized bed. Due to the higher degree of fluidization, circulating fluidized beds on the other hand have better conditions for a mass and heat transfer and allow the integration of a suspension heat exchanger, but are restricted in terms of their solids retention time due to the higher degree of fluidization.